1. Field of the Invention
The invention relates to methods and apparatus for estimating the volume of fluid absorbed by a patient during a surgical procedure.
2. Fluid Balance in Endoscopic Surgery
Endoscopic surgery is frequently performed on the uterus (transvaginally) and the prostate gland (transurethrally). These anatomic approaches generally require that an endoscope be inserted through an orifice into a body cavity and that excised tissue be removed from the surgical site through the same orifice. To maintain the surgeon's clear view of the surgical site and facilitate continuous removal of blood and small tissue fragments, a substantially continuous flush of electrically non-conducting irrigation fluid from an external reservoir is maintained through the endoscope. Such fluid is frequently formulated as a substantially isosmotic crystalloid solution comprising one or more nonelectrolytes such as glucose, urea, glycine, mannitol, or sorbitol.
While it is intended that incidental absorption of relatively small quantities of the above irrigation fluid will not harm the patient, absorption of larger quantities may well have adverse clinical consequences. For example, if irrigation fluid under a pressure head of approximately one meter is applied to the urethra and bladder of an adult patient undergoing transurethral resection of the prostate (TURP), one may estimate that about 10 to 30 ml of irrigation fluid will be absorbed per minute of resection time. On the other hand, the absorption of as much as 6 to 8 liters of irrigation fluid has been documented during TURP's extending over a period of 2 hours. Following absorption of such large amounts of fluid, serious problems of dilutional hyponatremia and/or over-hydration may be manifest.
The latter problem can be especially serious because a patient may retain only 20% to 30% of absorbed crystalloid solution within the intravascular space. The remaining fluid generally moves to the interstitial space, where it may substantially increase the likelihood of pulmonary and cerebral edema formation. Whether or not a given patient will actually develop pulmonary and cerebral edema, however, depends on several factors, including that patient's cardiovascular status, the amount and rate of onset of the irrigation fluid load, the initial fluid volume status of the patient, and the amount of blood loss during the operation.
Thus, careful monitoring of fluid intake by the patient in real time may be very helpful when combined with other preoperative and interoperative assessments. Note that while the total fluid intake by a patient during an endoscopic surgical procedure may often be approximated by considering only the volume of irrigation fluid absorbed, fluids administered intravenously may occasionally represent a clinically significant fraction of the total fluid intake. When this occurs, accurate incorporation of intravenously administered fluids within the overall fluid balance equation is relatively easy because such fluid is usually transported directly to the vascular system with little or no loss.
In contrast, estimation of irrigation fluid absorption is error-prone because as irrigation fluid drains continuously from both the endoscope and the body orifice in which it is inserted (i.e., the vagina or urethra as the case may be), fluid is commonly distributed over the surgical drapes, operating table and floor, as well as to containers resting on the floor. Fluid falling in the containers may subsequently be manually strained to recover any tissue which is to be retained for subsequent examination. Incidental absorption by and adsorption to various operating-room surfaces, as well as losses in handling due to spillage and splashing, make irrigation fluid recovery uncertain, thereby reducing the clinical usefulness of fluid absorption estimates.
Nevertheless, because pulmonary edema formation may be life-threatening and because serious electrolyte imbalances may result in seizures, coma or death of the patient, the surgeon must have sufficient warning of impending fluid overloads and/or electrolyte imbalances to take timely corrective action. Such warning may be provided by obtaining frequent estimates of arterial oxygen tension and/or serum electrolyte levels, but obtaining this information is unduly time-consuming and inconvenient. Blood for analysis of both oxygen tension and serum electrolyte levels is usually obtained by venipuncture, leading to increased patient discomfort and increased risk to medical personnel who handle the blood samples.
A more convenient method useful for estimating a patient's overall fluid volume status during endoscopic surgery would rely on preoperative patient assessments, combined with more accurate estimates of the amount of irrigation fluid absorbed intraoperatively. For the reasons described above, however, the latter estimates are not readily available in the typical clinical setting.